Книга: Iptables Tutorial 1.2.2

SCTP INIT ACK chunk

SCTP INIT ACK chunk

The INIT ACK chunk is sent in response to a INIT chunk and contains basically the same headers, but with values from the recipient of the original INIT chunk. In addition, it has two extra variable length parameters, the State Cookie and the Unrecognized Parameter parameters.

Type - bit 0-7. This header is always set to 2 for INIT ACK chunks.

Chunk flags - bit 8-15. Not used today. Might be applicable for change. See SCTP Common and generic headers for more information.

Chunk Length - bit 16-31. The chunk length is the length of the whole packet, including everything in the headers, and the optional parameters.

Initiate Tag - bit 32-63. The receiver of the Initiate Tag of the INIT ACK chunk must save this value and copy it into the Verification Tag field of every packet that it sends to the sender of the INIT ACK chunk. The Initiate Tag must not be 0, and if it is, the receiver of the INIT ACK chunk must close the connection with an ABORT.

Advertised Receiver Window Credit (a_rwnd) - bit 64-95. The dedicated buffers that the sender of this chunk has located for traffic, counted in bytes. The dedicated buffers should never be lowered to below this value.

Number of Outbound Streams - bit 96-111. How many outbound streams that the sending host wishes to create. Must not be 0, or the receiver of the INIT ACK should ABORT the association. There is no negotiation of the minimum number of outbound or inbound streams, it is simply set to the lowest that either host has set in the header.

Number of Inbound Streams - bit 112-127. How many inbound streams that the sending endpoint is willing to accept. Must not be 0, or the receiver of the INIT ACK should ABORT the association. There is no negotiation of the minimum number of outbound or inbound streams, it is simply set to the lowest that either host has set in the header.

Initial TSN - bit 128-159. This is set to the Initial Transmission Sequence Number (I-TSN) which will be used by the sending party in the association to start with.

After this point, the INIT ACK chunk continues with optional variable-length parameters. The parameters are exactly the same as for the INIT chunk, with the exception of the addition of the State Cookie and the Unrecognized Parameters parameter, and the deletion of the Supported Address Types parameter. The list in other words look like this:

Table 2-4. INIT ACK Variable Parameters

The State Cookie is used in INIT ACK to send a cookie to the other host, and until the receiving host has replied with a COOKIE ECHO chunk, the association is not guaranteed. This is to prevent basically the same as a SYN attack in TCP protocol.

Type - bit 0-15. Always set to 7 for all State Cookie parameters.

Length - bit 16-31. The size of the whole parameter, including the type, length and State Cookie field in bytes.

State Cookie - bit 31-n. This parameter contains a cookie of variable length. For a description on how this cookie is created, see the RFC 2960 - Stream Control Transmission Protocol document.

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